For the first time, Facebook will be the number two seller of digital ads in both the U.S. and worldwide, according to a new forecast by market research firm eMarketer. Pushed by mobile growth, the social network will move up from fourth place to sit between ad leader Google and third-place Microsoft.

In 2010, mobile contributed only 3% of global digital ad revenues. In the intervening years, it has rocketed to 22%, contributing $9.6 billion of the online ad sector. The advance of mobile use, with its rich ecosystem of applications and an increasing inventory of devices, has turned this once negligible slice of the ad market into a major factor.

Microsoft will drop one place this year, from second to third, according to the eMarketer forecast, losing .3% of market share to 5.9% and Yahoo will follow suit, declining a full point to 5.8% and dropping to fourth.

According to eMarketer, Facebook will take 7.4% of American digital advertising for a total of $3.17 billion this year, with Google capturing about a fourth of the market, for $17 billion in income.

Facebook will gain the #2 position globally as well as nationally, taking 5.64% of the worldwide market to Google’s 31.91%.

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